Composite transparent conductors and methods of forming the same

ABSTRACT

Composite transparent conductors are described, which comprise a primary conductive medium based on metal nanowires and a secondary conductive medium based on a continuous conductive film.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/106,193 filed Apr. 18, 2008 (U.S. Pat. No. 8,018,563); which claimsbenefit under 35 U.S.C. 119(e) of U.S. Provisional Patent ApplicationNo. 60/913,231, filed Apr. 20, 2007; these applications are incorporatedherein by reference in their entireties.

BACKGROUND

Technical Field

This disclosure is related to composite transparent conductors based onconductive nanostructures, and methods of forming the same.

Description of the Related Art

Transparent conductors refer to optically transparent, thin conductivefilms. They are widely used as transparent electrodes in flat panelelectrochomic displays such as liquid crystal displays, plasma displays,touch panels, electroluminescent devices and thin film photovoltaiccells, as anti-static layers and as electromagnetic wave shieldinglayers.

Conventional transparent conductors include vacuum deposited metaloxides, such as indium tin oxide (ITO). However, metal oxide films arecostly to fabricate because they require vacuum chambers, elevateddeposition temperatures and/or high annealing temperatures to achievehigh conductivity. Metal oxide films are also fragile and prone todamage even when subjected to minor physical stresses such as bending.

Conductive polymers have also been used as optically transparentelectrical conductors. However, they generally have lower conductivityvalues and higher optical absorption (particularly at visiblewavelengths) compared to the metal oxide films, and suffer from lack ofchemical and long-term stability.

Conductive nanostructures can form optically transparent conductivefilms due to their submicron dimensions. Copending and co-owned U.S.patent application Ser. Nos. 11/504,822, 11/871,767, and 11/871,721describe transparent conductors formed by networking anisotropicconductive nanostructures such as metal nanowires. Like the ITO films,nanostructure-based transparent conductors are particularly useful aselectrodes that can be coupled to thin film transistors inelectrochromic displays such as flat panel displays and touch screens.In addition, nanostructure-based transparent conductors are alsosuitable as coatings on color filters and polarizers, as polarizers, andso forth. The above copending applications are incorporated herein byreference in their entireties.

There is a need to provide cost-effective and high-performancenanostructure-based transparent conductors to satisfy the rising demandfor quality display systems.

BRIEF SUMMARY

Composite transparent conductors and their applications are described.

One embodiment describes a composite transparent conductor comprising: aprimary conductive medium including a plurality of metal nanowires or aplurality of metal nanotubes; and a secondary conductive medium coupledto the primary conductive medium, the secondary conductive mediumincluding a second type of nanostructures or a continuous conductivefilm.

Another embodiment describes a device, comprising a compositetransparent conductor including: a primary conductive medium including aplurality of metal nanowires or a plurality of metal nanotubes; and asecondary conductive medium coupled to the primary conductive medium,the secondary conductive medium being a continuous conductive film.

A further embodiment describes a liquid crystal display cell comprising:a first electrode; and a second electrode, wherein a vertical distancebetween the first electrode and the second electrode defines a cell gap;wherein the first electrode is a composite transparent conductorincluding a primary conductive medium and a secondary conductive medium,and wherein, the primary conductive medium includes metal nanowires ormetal nanotubes that have a mesh size on the order of the cell gap; andwherein, the secondary conductive medium is a continuous conductive filmor a conductive network of nanostructures having a mesh size of about ⅕to 1/100 of the cell gap.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

In the drawings, identical reference numbers identify similar elementsor acts. The sizes and relative positions of elements in the drawingsare not necessarily drawn to scale. For example, the shapes of variouselements and angles are not drawn to scale, and some of these elementsare arbitrarily enlarged and positioned to improve drawing legibility.Further, the particular shapes of the elements as drawn are not intendedto convey any information regarding the actual shape of the particularelements, and have been selected solely for ease of recognition in thedrawings.

FIG. 1 shows a film of metal nanowires at above electrical percolationlevel.

FIG. 2A shows a film of metal nanowires at below electrical percolationlevel.

FIG. 2B shows a composite transparent conductor comprising metalnanowires at below electrical percolation level and in combination witha continuous conductive film.

FIG. 2C shows a composite transparent conductor comprising metalnanowires at below electrical percolation level and in combination witha conductive film formed of a second type of anisotropic nanostructures.

FIG. 3A shows non-uniform electrical field localized between adjacentmetal nanowires.

FIG. 3B shows uniform electrical fields in the presence of continuousconductive films.

FIG. 4A-4C show embodiments of composite transparent conductors based onmetal nanowires and carbon nanotubes.

FIG. 5 shows a composite transparent conductor having two differenttypes of metal nanowires differs in their dimensions.

FIG. 6A-6B show embodiments of composite transparent conductors based onmetal nanowires and metal oxide films.

FIG. 6C shows schematically a pair of parallel resistors.

FIG. 7A-7B show embodiments of composite transparent conductors based onmetal nanowires and conductive polymer films.

FIG. 8 illustrates schematically a liquid crystal material positionedbetween two transparent electrodes.

FIG. 9 shows a device incorporating a composite transparent conductor.

DETAILED DESCRIPTION OF THE INVENTION

Generally speaking, a composite transparent conductor is a conductivefilm formed of at least two types of transparent conductive media. Morespecifically, the composite transparent conductor includes metallicanisotropic nanostructures (as described herein) as a primary conductivemedium, and a secondary conductive medium coupled to the primaryconductive medium. The secondary conductive medium is typically aconductive network of a second type of conductive nanostructures, or acontinuous conductive film formed of conductive polymers or metaloxides.

The electrical and optical properties of a composite transparentconductor are determined by factors such as the geometries,conductivities, optical properties, distribution and loading levels ofthe constituent conductive media.

In certain embodiments, a composite transparent conductor is a layeredstructure of discrete conductive films. In other embodiments, acomposite transparent conductor is a cohesive structure, in which two ormore types of conductive media (e.g., two or more types of conductivenanostructures) are fully integrated. Regardless of the structuralconfigurations, composite transparent conductors can exhibit propertiesbeyond the mere additive effects of the constituent conductive mediathrough a judicious selection of such constituent conductive media.

Conductive Nanostructures

In certain embodiments, the composite transparent conductor comprises atleast two types of nanostructures, one of which is directed to metallicanisotropic nanostructures. As used herein, “nanostructures” or“conductive nanostructures” generally refer to nano-sized structures, atleast one dimension of which is less than 500 nm, more preferably, lessthan 250 nm, 100 nm, 50 nm or 25 nm.

The nanostructures can be of any shape or geometry. In certainembodiments, the nanostructures are isotropically shaped (i.e., aspectratio=1). Typical isotropic nanostructures include nanoparticles. Inpreferred embodiments, the nanostructures are anisotropically shaped(i.e. aspect ratio≠1). As used herein, aspect ratio refers to the ratiobetween the length and the width (or diameter) of the nanostructure. Theanisotropic nanostructure typically has a longitudinal axis along itslength. Exemplary anisotropic nanostructures include nanowires andnanotubes, as defined herein.

The nanostructures can be solid or hollow. Solid nanostructures include,for example, nanoparticles and nanowires. “Nanowires” refer to solidanisotropic nanostructures, as defined herein. Typically, each nanowirehas an aspect ratio (length:diameter) of greater than 10, preferablygreater than 50, and more preferably greater than 100. Typically, thenanowires are more than 500 nm, or more than 1 μm, or more than 10 μm inlength.

Hollow nanostructures include, for example, nanotubes. “Nanotubes” referto hollow anisotropic nanostructures, as defined herein. Typically, thenanotube has an aspect ratio (length:diameter) of greater than 10,preferably greater than 50, and more preferably greater than 100.Typically, the nanotubes are more than 500 nm, or more than 1 μm, ormore than 10 μm in length.

The nanostructures can be formed of any conductive material. Mosttypically, the conductive material is metallic. The metallic materialcan be an elemental metal (e.g., transition metals) or a metal compound(e.g., metal oxide). The metallic material can also be a metal alloy ora bimetallic material, which comprises two or more types of metal.Suitable metals include, but are not limited to, silver, gold, copper,nickel, gold-plated silver, platinum and palladium. The conductivematerial can also be non-metallic, such as carbon or graphite (anallotrope of carbon).

As noted above, metallic anisotropic nanostructures are used as theprimary conductive medium in a composite transparent conductor. Apreferred type of anisotropic metallic nanostructures includes metalnanowires. Metal nanowires are nanowires formed of metal, metal alloys,plated metals or metal oxides. Suitable metal nanowires include, but arenot limited to, silver nanowires, gold nanowires, copper nanowires,nickel nanowires, gold-plated silver nanowires, platinum nanowires, andpalladium nanowires. Co-pending and co-owned U.S. application Ser. Nos.11/766,552, 11/504,822, 11/871,767, and 11/871,721 describe methods ofpreparing metal nanowires (e.g., silver nanowires) and methods offorming and patterning transparent conductors based on metal nanowires,the descriptions of which are incorporated herein by reference in theirentireties.

Another preferred type of anisotropic metallic nanostructures used inthe primary conductive medium includes metal nanotubes. Co-pending andco-owned U.S. Patent Application No. 61/031,643, filed Feb. 26, 2008,describes methods of preparing metal nanotubes (e.g., gold nanotubes)and methods of forming and patterning transparent conductors based onmetal nanotubes, the description of which is incorporated herein byreference in their entireties.

As will be discussed in more detail herein, the metallic anisotropicnanostructures, such as nanowires and nanotubes can be combined with asecondary conductive medium formed by a different type of conductivenanostructures. The secondary conductive medium can be any of followingnanostructures, including without limitation, carbon nanotubes, metallicnanowires (or nanotubes) different from the metallic nanowires (ornanotubes) that form the primary conductive medium, conductivenanoparticles and the like.

In certain specific embodiments, the conductive nanostructures formingthe secondary conductive medium are carbon nanotubes. Carbon nanotubesare also conductive anisotropic nanostructures. More specifically,“carbon nanotube” refers to a cylinder or tube of rolled up graphenesheet(s). Each graphene sheet comprises sp² hybridized carbon atoms.Carbon nanotubes can take the form of either single-walled ormulti-walled structures, or a mixture of both. A single-walled carbonnanotube (SWNT) is formed by a single rolled-up graphene sheet.Multi-walled carbon nanotubes (MWNTs) are two or more coaxially arrangedSWNTs nested in each other. Both SWNTs and MWNTs are known to showmetallic and conductive characteristics.

Carbon nanotubes are typically rigid structures of high aspect ratios.The lengths of SWNTs and MWNTs are usually well over 1 μm and diametersrange from about 1 nm (for SWNTs) to about 50 nm (for MWNTs). Typically,the aspect ratio of the carbon nanotubes is in the range of about10-100,000. More typically, the aspect ratio is in the range of about1,000-10,000. SWNTs are available commercially from Sigma-Aldrich (St.Louis, Mo.).

Carbon nanotubes may be optionally surface treated to preventaggregation. For example, hydrophilic functional groups may beincorporated onto the surface for better dispersion into the aqueousmedium. Various methods of surface treatments are described in Peng H.et al. Sidewall Carboxylic Acid Functionalization of Single-WalledCarbon Nanotubes, J. Am. Chem. Soc. 125, 15174-15182, 2003 and Liu J. etal. Fullerene Pipes, Science, 280, 1253-1256, 1998.

In further embodiments, the conductive nanostructures are conductivenanoparticles, including metallic nanoparticles such as silver, gold,copper, and nickel nanoparticles, and metal oxide nanoparticles suchindium tin oxide and doped zinc oxide nanoparticles. Non-metallicconductive nanoparticles include carbon black, graphene sheets, and thelike. These conductive nanoparticles are well known in the art.

Conductive nanostructures can achieve electrical conductivity throughcontinuous physical contact as well as electrical charge tunneling fromone nanostructure to another.

Primary Conductive Medium

Metal nanowires or metal nanotubes form the primary conductive medium.Suitable metal nanowires are nanowires formed of metal, metal alloys,plated metals or metal oxides. Suitable metal nanowires include, but arenot limited to, silver nanowires, gold nanowires, copper nanowires,nickel nanowires, gold-plated silver nanowires, platinum nanowires, andpalladium nanowires. Suitable metal nanotubes include gold nanotubes andthose described in co-pending U.S. Provisional Application No.61/031,643.

In various embodiments, the metal nanowires are about 5-100 μm long and5-100 nm in diameter (or cross-section). In certain embodiments, themetal nanowires are about 5-30 μm long and 20-80 nm in diameter. In apreferred embodiment, the metal nanowires (e.g., silver nanowires) areabout 20 μm long and 50 nm in diameter.

Suitable metal nanotubes have similar dimensions as those described formetal nanowires. For nanotubes, the diameter refers to the outerdiameter of the nanotubes.

Nanostructures form a conductive network through a percolation process.Percolative conductivity can be established when a conductive path isformed through interconnecting nanostructures. Sufficient nanostructuresmust be present to reach an electrical percolation threshold and becomeconductive. The electrical percolation threshold is therefore a criticalvalue related to the loading density or concentration of thenanostructures, above which long range connectivity can be achieved.Typically, the loading density refers to the number of nanostructuresper area, which can be represented by “number/μm²”.

As described in co-pending U.S. patent application Ser. No. 11/504,822,the higher the aspect ratio (length:diameter) of the nanostructures, thefewer nanostructures are needed to achieve percolative conductivity. Foranisotropic nanostructures, such as nanowires, the electricalpercolation threshold or the loading density is inversely related to thelength² of the nanowires. Co-pending and co-owned application Ser. No.11/871,053, which is incorporated herein by reference in its entirety,describes in detail the theoretical as well as empirical relationshipbetween the sizes/shapes of the nanowires and the surface loadingdensity at the percolation threshold.

FIG. 1 shows schematically a conductive network 10 formed by nanowires20 at above an electrical percolation threshold. Conductive paths areformed by interconnecting nanowires (e.g., a path can be traced from oneend of the network to the other through connecting nanowires). Anelectrical current can therefore be carried across the nanowire network10.

As used herein, “conductive network” or “network” refers to aninterconnecting network formed by conductive nanostructures above anelectrical percolation threshold. Typically, a conductive networksurface resistivity (or “sheet resistance”) of no higher than 10⁸ohms/square (also referred to as “Ω/□”). Preferably, the surfaceresistivity is no higher than 10⁴ Ω/□, 3,000Ω/□, 1,000 Ω/□ or 100 Ω/□.Typically, the surface resistivity of a conductive network formed bymetal nanowires is in the ranges of from 10 Ω/□ to 1000 Ω/□, from 100Ω/□ to 750 Ω/□, 50 Ω/□ to 200 Ω/□, from 100 Ω/□ to 500 Ω/□, or from 100Ω/□ to 250 Ω/□, or 10 Ω/□ to 200 Ω/□, from 10 Ω/□ to 50 Ω/□, or from 1Ω/□ to 10 Ω/□.

Also shown in FIG. 1, the networking nanowires define inter-wire spaces30. At above the percolation threshold, the size of the inter-wire space(also referred to as “mesh size”) correlates to the conductivity of thenetwork. Typically, smaller mesh size means more densely distributednanowires, which in turn correspond to higher conductivity.

Mesh size can also be used as an indicator of the surface loading level.For example, for nanowires of a given length, lower surface loading willresult in larger mesh size. When the mesh size is above certainthreshold value, the nanowires can become too far apart such thatpercolation is no longer possible and the inter-wire spaces effectivelybecome insulators. FIG. 2A shows a film 12 in which nanowires 20 are atan insufficient density to form a complete network. The inter-wirespaces 30 become insulating. Stately differently, due to the lowerdensity of the nanowires as compared to that in FIG. 1, the mesh sizehas enlarged and the conductivity between nanowires disrupted.

Secondary Conductive Medium as Fillers

In a composite transparent conductor, even if the metal nanowires are ata loading level below the electrical percolation threshold, conductivitycan be achieved in the presence of the secondary conductive medium.While the metal nanowires of the primary conductive medium can be, invarious embodiments, percolative or not percolative, the presence of thesecond conductive medium provides unexpected or synergistic propertiesin the composite transparent conductor.

In certain embodiments, the secondary conductive medium includesnanostructures of a different material, dimension, geometry or structurefrom those of the metal nanowires that form the primary conductivemedium. For example, the secondary conductive medium may include,without limitation, carbon nanotubes, metal nanotubes, nanoparticles,and metal nanowires of a different dimension or material.

In other embodiments, the secondary conductive medium can be acontinuous conductive film. As used herein, “continuous conductive”refers to an uninterrupted and uniform conductive path across a thinlayer (e.g., across a surface or in-plane), in which the electricalconductivity is established by continuous physical contacts of theconductive medium. Examples of continuous conductive films include,without limitation, sputtered or deposited metal oxide films, conductivepolymer films, and the like.

In one respect, the second conductive medium serves to fill in theinter-wire space of a nanowire film. FIG. 2B shows a compositetransparent conductor 34, in which a continuous conductive film 40 addedto the nanowires 20 of FIG. 2A. The continuous conductive film fills theinsulating spaces 30 and effectively eliminates the mesh size.

FIG. 2C shows another composite transparent conductor 44, in which aplurality of a second type of anisotropic nanostructures 48 is alsopresent. The anisotropic nanostructures 48 are shown as having muchhigher aspect ratio than the nanowires 20. As shown, the inter-wirespace 30 is effectively reduced due to the more efficient connectivityby the longer nanostructures 48.

As shown in FIGS. 2B and 2C, the combined effects of the nanowires andthe secondary conductive medium establish conductivity even though theprimary conductive medium does not necessarily reach the electricalpercolation threshold.

In a further respect, the presence of the second conductive medium thatfills the inter-wire space also serves to equalize the electricalpotential distribution in a give transparent conductor. In addition,when two electrodes are spaced apart and an electrical potential isapplied, an electrical field is created between the space of the twoelectrodes. Employing composite transparent conductor as the electrodesserves to enhance the uniformity of the electrical field FIG. 3A showselectric field lines between a top conductive film 50 and a bottomconductive film 54. Both conductive films 50 and 54 are based onnanowires alone. The top conductive film 50 comprises nanowires 50 a(shown in cross-sectional view) distributed on a top substrate 50 b.Likewise, the bottom conductive film 54 comprises nanowires 54 a (alsoshown in cross-sectional view) distributed on a bottom substrate 54 b.The electrical field (shown schematically as line 58) begins from, as anexample, nanowires 50 a and end at 54 a. Due the inter-wire spaces(e.g., 62 and 66) between nanowires in each electrode, the lines 58 areconcentrated near the opposing wires. FIG. 3B shows a secondaryconductive medium, for example, continuous films 70 and 74 fill in theinter-wire spaces 62 and 66, respectively. As a result, the electricalfield, represented by lines 78, is more uniformly distributed.

As the primary conductive medium, highly conductive metal nanowirestypically bear the majority of the electrical current in a compositetransparent conductor. The secondary conductive medium, although notburdened with current-carrying, nonetheless can form a conductive layerthat fills in the space between the metal nanowires. For purpose of thisdescription, the secondary conductive medium forms a conductive layerthat has a surface resistivity (or “sheet resistance”) of no higher than10⁸ ohms/square (also referred to as “Ω/□”). Preferably, the surfaceresistivity is no higher than 10⁴ Ω/□, 3,000 Ω/□, 1,000 Ω/□ or 100 Ω/□.Typically, the sheet resistance of a continuous conductive film is inthe ranges of from 10 Ω/□ to 1000 Ω/□, from 100 Ω/□ to 750 Ω/□, 50 Ω/□to 200 Ω/□, from 100 Ω/□ to 500 Ω/□, or from 100 Ω/□ to 250 Ω/□, or 10Ω/□ to 200 Ω/□, from 10 Ω/□ to 50 Ω/□, or from 1 Ω/□ to 10 Ω/□.

In various embodiments, the conductive layer formed by the secondconductive medium is optically clear, as defined herein. Further, thepresence of the secondary conductive medium may lead to an overallreduction in light scattering. Metal nanowires are reflectivestructures, which can cause depolarization due to light scattering andreflectivity. Depolarization is one of the main factors that contributeto reducing the contrast ratio in a transparent conductor film, which istypically in a light path of a display device (e.g., flat paneldisplay). Lower contrast ratio tends to adversely affect the imagequality of the display device. See, e.g. co-pending U.S. ProvisionalApplication No. 61/031,643. In a transparent conductor film solelyformed of nanowires, a reduction in the number of the nanowires couldresult in a reduction in the light scattering, but potentially at theexpense of a loss in conductivity. The composite film according to thisembodiment allows for a reduction in reflectivity by employing fewernanowires without necessarily causing a decrease in conductivity due tothe supplemental connectivity provided by the second conductive medium.

Moreover, by selecting nanostructures of an appropriate material (e.g.,less reflective or non-reflective), a particular dimension (e.g.,nanostructures having smaller diameters or cross-sections cause lesslight scattering), a particular geometry (e.g., nanotubes cause lesslight scattering than nanowires of the same outer diameter), it ispossible to customize composite transparent conductor with optimizedoptical properties.

Typically, in various embodiments, the conductive layer formed by thesecond conductive medium is about 100 nm to 200 nm thick, or 50 nm to100 nm thick, or 150 nm to 200 nm thick.

Composite Transparent Conductor

Thus, a composite transparent conductor comprises metal nanowires as aprimary conductive medium and a secondary conductive medium coupled tothe primary conductive medium. As used herein, “coupled” refers to theproximate association between the two conductive media and includesphysical contact, electrical connection and so forth.

The combined conductive media in the composite provide unexpectedattributes or enhanced properties than the sum of the individualconductive medium. As will be described in more detail herein, thesynergistic improvements of the composite transparent conductor include,but are not limited to, more equalized electrical potential in acomposite transparent conductor, a more uniform electrical field betweentwo electrodes form by the composite transparent conductor, higherconductivity, better durability, higher contrast ratio and so forth. Inaddition, when combining nanowires with a judicious selection of thesecondary conductive medium, the overall fabrication cost can be reducedwithout compromising the performance standard of the compositetransparent conductor.

The following specific embodiments describe composite transparentconductors based on metal nanowires as a primary conductive medium andvarious secondary conductive media.

1. Carbon Nanotube Film as the Secondary Conductive Medium

In another embodiment, the composite transparent conductor comprises aplurality of metal nanowires combined with a secondary conductivemedium, wherein the secondary conductive medium is a continuousconductive film formed of carbon nanotubes (CNT).

FIG. 4A shows a composite transparent conductor 140 including a nanowirelayer 144 and an underlying CNT layer 148 formed on a substrate 152. TheCNTs form a conductive film underlying the nanowire layer. FIG. 4B showsa composite transparent conductor 150 having a reverse arrangement ofthe constituent films, in which the nanowire layer 144 underlies the CNTlayer 148. In both FIGS. 4A and 4B, the constituent films can bedeposited sequentially. Alternatively, nanowires and CNTs can also beco-deposited simultaneously and forms a fully integrated conductivefilm. FIG. 4C shows a composite transparent conductor 160 having aconductive layer 164 in which nanowires 168 and CNTs 172 are fullyintegrated to afford a cohesive structure.

The composite films shown in FIGS. 4A-4C provide long range connectivitythat relies on the complementary properties of the highly conductivemetal nanowires to carry an electrical current, and the filling effectsof the conductive CNT film. Because the CNTs have much lower specificweight (about 1.7-1.9 g/cm³) compared to metal nanowires (about 10.5g/cm³ for silver nanowires), at a given loading level, CNTs can form aconductive film with smaller mesh sizes as compared to metal nanowires.Thus, composite transparent conductors having a CNT layer can alsoimproves the uniformity of the electrical potential of the compositefilm when connected to a power source.

In addition, the CNTs are black and have very narrow dimension (i.e.,their diameters or cross-sectional areas are typically less than 2 nm),which are desirable conditions for reducing light scattering andimproving contrast ratio. As a result, the combined conductive mediabased on CNTs and metal nanowires reduce the overall reflectivity at agiven conductivity.

Moreover, a composite film based on CNTs and nanowires are particularlysuitable as via contacts. As used herein, “via” refers to the connectionbetween two conductors, typically through a dielectric layer. Asdiscussed, because the CNTs have much lower specific weight than that ofthe metal nanowires, the loading density of the CNTs can be much higherper unit area than metal nanowires of the same weight. This can beadvantageously applied to via contacts, which are burdened withsupporting high current densities in a confined area (about 5-10microns). The larger density of the CNTs can effectively carry theadditional current and prevent potential damages to the metal nanowires.

In certain embodiments, a third conductive medium can be furtherincorporated into the composite transparent conductor. As used herein,“second type of nanostructures” and “third type nanostructures”specifically refer to nanostructures that are different from each otheras well as from the metal nanowires or metal nanotubes that form theprimary conductive medium in at least one respect, such as the material,the dimension, the shape, or the geometry of the nanostructure.

Suitable third conductive medium includes conductive nanostructures suchas conductive nanoparticles, conductive nanostructures of a differentmaterial, dimension or geometry from those of the metal nanowires of theprimary conductive medium. For example, conductive nanoparticles can bemetallic nanoparticles, metal oxide nanoparticles, carbon blacks, and acombination thereof. Conductive nanostructures can be nanowires of adifferent metal, nanotubes, or nanowires of a higher aspect ratio or asmaller cross-section. The third type of conductive nanostructures thatare distributed throughout the composite transparent conductor cansupplement the filling effect of the CNTs and contribute to rendering amore equalized electrical potential across the composite transparentconductor.

Typically, the composite transparent conductor based on a combination ofmetal nanowires (e.g., silver nanowires) and a CNT film has a lighttransmission of at least 50%, at least 60%, at least 70%, or at least80%, or at least 85%, or at least 90%, or at least 95% (using air asreference).

Typically, the composite transparent conductor based on a combination ofmetal nanowires (e.g., silver nanowires) and a CNT film has a sheetresistance in the range from 1-10⁸Ω/□, depending on the end applicationof the composite transparent conductor. More typically, the sheetresistance is in the ranges of from 10 Ω/□ to 1000 Ω/□, from 100 Ω/□ to750 Ω/□, 50 Ω/□ to 200 Ω/□, from 100 Ω/□ to 500 Ω/□, or from 100 Ω/□ to250 Ω/□, or 10 Ω/□ to 200 Ω/□, from 10 Ω/□ to 50 Ω/□, or from 1 Ω/□ to10 Ω/□.

In preferred embodiments, the composite transparent conductor based on acombination of metal nanowires (e.g., silver nanowires) and a CNT filmhas a light transmission higher than 85% and a sheet resistance of lessthan 1000 Ω/□. In other embodiments, the composite transparent conductorbased on a combination of metal nanowires (e.g., silver nanowires) and aCNT film has a light transmission higher than 95% and a sheet resistanceof less than 500 Ω/□. In other embodiments, the composite transparentconductor based on a combination of metal nanowires (e.g., silvernanowires) and a CNT film has a light transmission higher than 90% and asheet resistance of less than 100 Ω/□. In other embodiments, thecomposite transparent conductor based on a combination of metalnanowires (e.g., silver nanowires) and a CNT film has a lighttransmission higher than 85% and a sheet resistance of less than 50 Ω/□.

2. Other Types of Nanostructures as the Secondary Conductive Medium

Nanostructures other than CNTs are also suitable as the secondconductive medium. In certain embodiments, the conductive nanostructuresare metal nanowires of a different material or dimension from the metalnanowires that form the primary conductive medium. For example,nanowires formed of a less reflective metal or having a less reflectiveoxide sheath can be used to reduce light scattering without compromisingthe overall conductivity of the composite transparent conductor.Further, nanowires having a smaller diameter (i.e., cross-sectionalarea) compared to that of the metal nanowires of the primary conductivemedium can also reduce light scattering.

FIG. 5 shows a composite transparent conductor 170 comprising a firsttype of nanowires 174 as the primary conductive medium and a second typeof nanowires 188. The second type of nanowires 188 has a much smallerdiameter than that of the first type of nanowires 174. As a result, thesecondary conductive medium not only facilitates the conductivity of thecomposite transparent conductor by filling in the inter-wire space 182but also do not substantially contribute to light scattering due totheir narrow dimensions.

In various other embodiments, the second type of nanostructures can bemetal nanotubes, conductive nanoparticles (such as carbon blacks andmetal or metal oxide nanoparticles), and the like.

3. Metal Oxide Film as the Secondary Conductive Medium

In one embodiment, the composite transparent conductor comprises aplurality of metal nanowires combined with a secondary conductivemedium, wherein the secondary conductive medium is a conductive metaloxide film. Conductive metal oxides such as indium tin oxide (ITO) arewell known in the art. Sputtered ITO films have been conventionallyapplied to devices that employ transparent conductors. However, the ITOfilms are limited in their applications due to their brittleness and lowtolerance to stress. Even minute fractures in an ITO film can cause arapid loss of conductivity.

Combining a metal nanowire-based film and an ITO film affords acomposite film having synergistic advantages. FIG. 6A shows a compositefilm 186 comprises an ITO film 188 on a substrate 110 (e.g., glass), anda nanowire film 192 positioned on top of the ITO film 188, the nanowirefilm 192 comprising nanowires 194.

In one embodiment, the loading density of the nanowires 194 is below theelectrical percolation threshold. Nevertheless, surface conductivity canbe established in the composite film 186 by the combination of thenanowires and the underlying ITO film 188. As discussed, the ITO film iscapable of filling in any insulating gap between the nanowires.

FIG. 6B shows a composite film 196 having an alternative arrangement ofthe nanowire-based film and the ITO film. As shown, the nanowire film192 is first deposited on the substrate 110. The ITO film 188 issputtered on top of the nanowire film 192. As in FIG. 6A, the nanowires194 do not necessarily form a conductive network themselves.Nevertheless, in-plane conductivity can be established in the compositefilm 196 by the combination of the nanowires and the overlying ITO film188.

As shown, conductivity throughout the composite film, including surfaceand in-plane conductivity, can be superior to that of either constituentfilm alone, i.e., the nanowire-based film and the ITO film.Advantageously, the constituent films complement each other tosynergistically provide properties that are more than the mere additiveeffects of the constituent films. For example, due to the presence of acontinuous ITO film, when connected to a voltage source, the compositefilm has a more uniform electrical potential than that of a transparentconductor based on nanowires alone (see, also, FIG. 2B). On the otherhand, the nanowires allow for certain degrees of flexing in thecomposite film without causing loss in conductivity. For example, thenanowires can bridge minor fractures within the bulk of the ITO film andmaintain conductivity, thus preventing potential failures in thecomposite film when in physical stress.

In addition, because of the high conductivity of the nanowires, theconductivity of the composite film can be much higher compared to thatof a pure ITO film at the same thickness. It is therefore possible toproduce a composite film that has a thinner ITO film as a constituentthan a pure ITO film, yet is capable of reaching the same level ofconductivity as the pure, thicker ITO film. Reducing the thickness of anITO film can directly result in a reduction in fabrication cost, andresult in an ITO film that is less prone to fracture.

Moreover, although the constituent films of FIGS. 6A and 6B are in anarrangement that resembles two parallel resistors, it is observed thatthe resistivity of the composite film can be lower than the resistivityexpected for the parallel resistors (see, also, Example 4). FIG. 6Cschematically shows two parallel resistors 198 (resistivity R1) and 199(resistivity R2). As known, the overall resistivity R of a set ofparallel resistors is:R=(R1×R2)/(R1+R2)

Example 4 measures the resistivity of the composite film formed by anITO film having a resistivity of 250 Ω/□ and a nanowire-based filmhaving a resistivity of about 250 Ω/□. If these two constituent filmsare merely parallel resistors, the overall resistivity would have beenabout 125 Ω/□. However, it was observed that the resistivity of thecomposite film was in the range of about 50-80 Ω/□, which was much lowerthan the expected resistivity of ITO film (250 Ω/□) and nanowire film(250 Ω/□) as parallel resistors.

Optically, the composite film can be less reflective than ananowire-based film alone at a given conductivity level. As discussed,in a transparent conductor film solely formed of nanowires, a reductionin the number of the nanowires could result in a reduction in lightscattering in the transparent conductor, but potentially at the expenseof a loss in conductivity. The composite film according to thisembodiment allows for a reduction in light scattering by employing fewernanowires without necessarily causing a decrease in conductivity due tothe supplemental connectivity provided by the ITO film.

Other metal oxide films can be used in the place of the ITO film ofFIGS. 6A and 6B. Exemplary metal oxide films include doped zinc oxidefilm, fluorine doped tin oxide film, aluminum doped zinc oxide film,Zn₂SnO₄, ZnSnO₃, MgIn₂O₄, GaInO₃, (Ga₂In)₂O₃, Zn₂In₂O₅, In₄Sn₃O₁₂ and soforth. Crawford, G. P., Flexible Flat Panel Display (John Wiley andSons, 2005).

Typically, the composite transparent conductor based on a combination ofmetal nanowires (e.g., silver nanowires) and a metal oxide film has alight transmission of at least 50%, at least 60%, at least 70%, or atleast 80%, or at least 85%, or at least 90%, or at least 95% (using airas reference).

Typically, the composite transparent conductor based on a combination ofmetal nanowires (e.g., silver nanowires) and a metal oxide film has asheet resistance in the range from 1-10⁸ Ω/□, depending on the endapplication of the composite transparent conductor. More typically, thesheet resistance is in the ranges of from 10 Ω/□ to 1000 Ω/□, from 100Ω/□ to 750 Ω/□, 50 Ω/□ to 200 Ω/□, from 100 Ω/□ to 500 Ω/□, or from 100Ω/□ to 250 Ω/□, or 10 Ω/□ to 200 Ω/□, from 10 Ω/□ to 50 Ω/□, or from 1Ω/□ to 10 Ω/□.

In preferred embodiments, the composite transparent conductor based on acombination of metal nanowires (e.g., silver nanowires) and a metaloxide film has a light transmission higher than 85% and a sheetresistance of less than 1000 Ω/□. In other embodiments, the compositetransparent conductor based on a combination of metal nanowires (e.g.,silver nanowires) and a metal oxide film has a light transmission higherthan 95% and a sheet resistance of less than 500 Ω/□. In otherembodiments, the composite transparent conductor based on a combinationof metal nanowires (e.g., silver nanowires) and a metal oxide film has alight transmission higher than 90% and a sheet resistance of less than100 Ω/□. In other embodiments, the composite transparent conductor basedon a combination of metal nanowires (e.g., silver nanowires) and a metaloxide film has a light transmission higher than 85% and a sheetresistance of less than 50 Ω/□.

4. Conductive Polymer Film as the Secondary Conductive Medium

In another embodiment, the composite transparent conductor comprises aplurality of metal nanowires combined with a secondary conductivemedium, wherein the secondary conductive medium is a continuous polymerfilm.

Certain polymers are conductive due to electronic delocalizationthroughout a conjugated backbone of continuous overlapping orbitals. Forexample, polymers formed of alternating single and double carbon-carbonbonds can provide a continuous path of overlapping p orbitals in whichthe electrons can occupy.

Common classes of organic conductive polymers include, with limitation,poly(acetylene)s, poly(pyrrole)s, poly(thiophene)s, poly(aniline)s,poly(fluorene)s, poly(3-alkylthiophene)s,poly(3,4-ethylenedioxythiophene), also known as PEDOT,polytetrathiafulvalenes, polynaphthalenes, polyparaphenylene,poly(paraphenylene sulfide), and poly(paraphenylene vinylene)s.

Although a conductive polymer film alone is typically not conductive orphysically robust enough to function as a transparent conductor in adisplay device, the conductive polymer film can be combined or dopedwith metal nanowires to form a composite transparent conductor. Thecomposite transparent conductor can rely on the metal nanowires as theprinciple current-carrying medium and the conductive polymer film as afiller to even out the electrical field. In addition, the metalnanowires can also reinforce and strengthen the mechanical properties ofthe conductive polymer films.

Optically, the conductive polymer film can also adjust the absorptioncharacteristics of the composition film.

FIG. 7A shows a composite film 200 comprises a conductive polymer film204 on a substrate 110 (e.g., glass), and a nanowire film 220 positionedon top of the conductive polymer film 204.

FIG. 7B shows a composite film 230 having an alternative arrangement ofthe nanowire-based film and the conductive polymer film. As shown, thenanowire film 220 is first deposited on the substrate 110. Theconductive polymer film 104 is deposited on top of the nanowire film220. As in FIG. 6A, the nanowires 224 do not necessarily form aconductive network themselves. Nevertheless, in-plane conductivity canbe established in the composite film 230 by the combination of thenanowires and the overlying conductive polymer film 204.

In an alternative embodiment, the metal nanowires are first deposited onthe substrate and form a conductive network. The conductive polymer filmcan be formed in situ using the metal nanowires network as an electrode.An example of suitable conductive polymers that can be formed in situ ispolypyrrole. More specifically, using nanowire-based conductive networkas an electrode (i.e., an anode), pyrrole monomers can electrochemicallypolymerize and form a coating on the conductive network. The conductivepolymer film can also be formed chemically in the presence of anoxidative agent according to known methods in the art. The resultingcomposite transparent conductor features nanowires embedded in aconductive polymer film.

Typically, the composite transparent conductor based on a combination ofmetal nanowires (e.g., silver nanowires) and a conductive polymer filmhas a light transmission of at least 50%, at least 60%, at least 70%, orat least 80%, or at least 85%, or at least 90%, or at least 95% (usingair as reference).

Typically, the composite transparent conductor based on a combination ofmetal nanowires (e.g., silver nanowires) and a conductive polymer filmhas a sheet resistance in the range from 1-10⁸ Ω/□, depending on the endapplication of the composite transparent conductor. More typically, thesheet resistance is in the ranges of from 10 Ω/□ to 1000 Ω/□, from 100Ω/□ to 750 Ω/□, 50 Ω/□ to 200 Ω/□, from 100 Ω/□ to 500 Ω/□, or from 100Ω/□ to 250 Ω/□, or 10 Ω/□ to 200 Ω/□, from 10 Ω/□ to 50 Ω/□, or from 1Ω/□ to 10 Ω/□.

In preferred embodiments, the composite transparent conductor based on acombination of metal nanowires (e.g., silver nanowires) and a conductivepolymer film has a light transmission higher than 85% and a sheetresistance of less than 1000 Ω/□. In other embodiments, the compositetransparent conductor based on a combination of metal nanowires (e.g.,silver nanowires) and a conductive polymer film has a light transmissionhigher than 95% and a sheet resistance of less than 500 Ω/□. In otherembodiments, the composite transparent conductor based on a combinationof metal nanowires (e.g., silver nanowires) and a conductive polymerfilm has a light transmission higher than 90% and a sheet resistance ofless than 100 Ω/□. In other embodiments, the composite transparentconductor based on a combination of metal nanowires (e.g., silvernanowires) and a conductive polymer film has a light transmission higherthan 85% and a sheet resistance of less than 50 Ω/□.

Electrical and Optical Properties

As discussed herein, the combined conductive media in the compositetransparent conductor provide unexpected attributes or enhancedproperties than the sum of the individual conductive medium. Thesesynergistic improvements of the composite transparent conductor include,but are not limited to, more uniform electrical potential (whenconnected to a power source), higher conductivity, better durability,higher contrast ratio and so forth.

Typically, the composite transparent conductor based on a combination ofmetal nanowires (e.g., silver nanowires) and a secondary conductivemedium has a light transmission of at least 50%, at least 60%, at least70%, or at least 80%, or at least 85%, or at least 90%, or at least 95%(using air as reference). Haze is an index of light scattering. Itrefers to the percentage of the quantity of light separated from theincident light and scattered during transmission (i.e., transmissionhaze). Unlike light transmission, which is largely a property of themedium, haze is often a production concern and is typically caused bysurface roughness and embedded particles or compositionalheterogeneities in the medium. In various embodiments, the haze of thetransparent conductor is no more than 10%, no more than 8%, no more than5%, no more than 3% or no more than 1%.

Typically, the composite transparent conductor based on a combination ofmetal nanowires (e.g., silver nanowires) and a secondary conductivemedium has a sheet resistance in the range from 1-10⁸ Ω/□, depending onthe end application of the composite transparent conductor. Moretypically, the sheet resistance is in the ranges of from 10 Ω/□ to 1000Ω/□, from 100 Ω/□ to 750 Ω/□, 50 Ω/□ to 200 Ω/□, from 100 Ω/□ to 500Ω/□, or from 100 Ω/□ to 250 Ω/□, or 10 Ω/□ to 200 Ω/□, from 10 Ω/□ to 50Ω/□, or from 1 Ω/□ to 10 Ω/□.

In preferred embodiments, the composite transparent conductor based on acombination of metal nanowires (e.g., silver nanowires) and secondaryconductive medium has a light transmission higher than 85% and a sheetresistance of less than 1000 Ω/□. In other embodiments, the compositetransparent conductor based on a combination of metal nanowires (e.g.,silver nanowires) and a secondary conductive medium has a lighttransmission higher than 95% and a sheet resistance of less than 500Ω/□. In other embodiments, the composite transparent conductor based ona combination of metal nanowires (e.g., silver nanowires) and asecondary conductive medium has a light transmission higher than 90% anda sheet resistance of less than 100 Ω/□. In other embodiments, thecomposite transparent conductor based on a combination of metalnanowires (e.g., silver nanowires) and a secondary conductive medium hasa light transmission higher than 85% and a sheet resistance of less than50 Ω/□.

The composite transparent conductor described herein can possesselectrical and optical properties suitable as electrodes in flat paneldisplays. A typical range of sheet resistances for transparentelectrodes in a flat panel display is about 10-100 Ω/□ with atransmission of the layer of higher than 87% (when used glass asreference) or higher than 95% (when using air as reference).

Moreover, when used as electrodes, the composite transparent conductorsprovide uniform electrical fields that are particularly advantageous ina liquid crystal display (LCD). FIG. 8 shows schematically a LCD set-up250, in which pixel electrode 254 and counter electrode 260 are spacedapart by about 3-5 μm, also referred to as “cell gap”. Liquid crystalcells 270 are positioned between the two electrodes. Simply stated, anLCD operates when liquid crystal molecules confined in a cell changetheir conformation in response to an applied electric field generatedbetween two electrodes.

Using transparent conductor electrodes formed solely of nanowires, atthe desired level of conductivity and light transmission, the spacesbetween the nanowires may be comparable to the liquid crystal cell cap(i.e., “cell gap”). Thus, it is possible that not all of the liquidcrystal molecules in the cell will be driven by the same electricalfield (both magnitude and direction), causing undesired localnonuniformity in the optical properties of the cell.

Employing composite transparent conductors as the electrodes, however,effectively reduces or eliminates the space between the nanowires.Typically, the mesh size between the nanowires should be less than ⅕ ofthe liquid crystal cell gap. More typically, the mesh size should beless than 1/10 or 1/100 of the cell gap. The presence of the secondaryconductive medium enables a uniform electrode field be applied acrossthe liquid crystal cell, resulting in a uniform orientation of theliquid crystal molecules and therefore a homogeneous optical response.

As shown, if an electrode in the LCD cell comprises only nanowires, asurface loading level to afford mesh sizes of ⅕- 1/100 of cell gap willresult in poor optical properties, including high haze and low contrastratio. However, when composite transparent conductor is used as theelectrode, the metal nanowires (or metal nanotubes) of the primaryconductive medium can maintain a mesh size on the order of the cell gap,while the secondary conductive medium effectively reduces the mesh sizeto about ⅕- 1/100 of the cell gap, or eliminates the mesh size, as inthe case of the continuous conductive film. The resulting LCD cell willhave improved cell performance as the optical properties of theelectrodes improve.

Thus, one embodiment provides a liquid crystal cell a first electrode;and a second electrode, wherein a vertical distance between the firstelectrode and the second electrode defines a cell gap; wherein the firstelectrode is a composite transparent conductor including a primaryconductive medium and a secondary conductive medium, and wherein, theprimary conductive medium includes metal nanowires or metal nanotubesthat have a mesh size on the order of the cell gap; and wherein, thesecondary conductive medium is a continuous conductive film or aconductive network of nanostructures having a mesh size of about ⅕ to1/100 of the cell gap.

Typically, the cell gap is about 3-5 μm. In certain embodiments, theconductive network of nanostructures have a mesh size of about ⅕ to 1/10of the cell gap, or about 1/10 to 1/100 of the cell gap.

Any of the above described composite transparent conductor can be usedas the first electrode of the liquid crystal cell. For example, invarious embodiments, the primary conductive medium can metal nanowires(e.g., silver nanowires) or metal nanotubes (e.g., gold nanotubes). Inpreferred embodiments, the metal nanowires or metal nanotubes are 20-80nm in diameter (outer diameter for nanotubes) and 5-30 μm long.

The secondary conductive medium may include a conductive network ofcarbon nanotubes, metal nanowires different from the metal nanowires ofthe primary conductive medium, or metal nanotubes different from themetal nanotubes of the primary conductive medium.

Alternatively, the secondary conductive medium can be a continuousconductive film such as a metal oxide film (e.g., ITO film) or aconductive polymer film (e.g., PEDOT film).

In a further embodiment, the second electrode can also be a compositetransparent conductor, as described herein.

In certain embodiments, the first electrode has a light transmission of80-95%.

As discussed, the composite transparent conductors can be designed toreduce undesirable level of scattering typically associated with metalnanowires. Because the secondary conductive medium is current-carrying,fewer nanowires are required to achieve a given conductivity. Inaddition, the secondary conductive medium described here is typicallynon-reflective, low-reflective, or comprises nanostructures having smallscattering cross-sections; as a result, the overall scatteringdiminishes because fewer nanowires are present.

Additional Layers

In a further embodiment, an inert layer of overcoat can be deposited tostabilize and protect the composite transparent conductor. The overcoatcan also provide favorable optical properties, such as anti-glare andanti-reflective properties, which serve to further reduce thereflectivity of the nanoparticles.

Thus, the overcoat can be one or more of a hard coat, an anti-reflectivelayer, a protective film, a barrier layer, and the like, all of whichare extensively discussed in co-pending application Ser. Nos. 11/871,767and 11/504,822.

Examples of suitable hard coats include synthetic polymers such aspolyacrylics, epoxy, polyurethanes, polysilanes, silicones,poly(silico-acrylic) and so on. Suitable anti-glare materials are wellknown in the art, including without limitation, siloxanes,polystyrene/PMMA blend, lacquer (e.g., butylacetate/nitrocellulose/wax/alkyd resin), polythiophenes, polypyrroles,polyurethane, nitrocellulose, and acrylates, all of which may comprise alight diffusing material such as colloidal or fumed silica. Examples ofprotective film include, but are not limited to: polyester, polyethyleneterephthalate (PET), polybutylene terephthalate, polymethyl methacrylate(PMMA), acrylic resin, polycarbonate (PC), polystyrene, triacetate(TAO), polyvinyl alcohol, polyvinyl chloride, polyvinylidene chloride,polyethylene, ethylene-vinyl acetate copolymers, polyvinyl butyral,metal ion-crosslinked ethylene-methacrylic acid copolymers,polyurethane, cellophane, polyolefins or the like; particularlypreferable are PET, PC, PMMA, or TAO.

Patterning

The composite transparent conductor described herein can be patterneddepending on their end uses. Any known methods in the art and all of thepatterning methods described in co-owned and co-pending U.S. patentapplication Ser. Nos. 11/504,822, 11/871,767, can be used to pattern thecomposite transparent conductor.

Applications of the Composite Transparent Conductor

The composite transparent conductor described herein can be used asfunctional films such as transparent electrodes, polarizers, colorfilters in a wide variety of devices, including all the devices thatcurrently makes use of metal oxide films (e.g., ITO). FIG. 7 showsschematically a device 250 including a composite transparent conductor254 in a housing 258. The composite transparent conductor can be any ofthe above described configurations or combinations of the primaryconductive medium (i.e., a plurality of metal nanowires) and the secondconductive medium (i.e., a continuous conductive film).

Examples of suitable devices include flat panel displays such as LCDs,plasma display panels (PDP), coatings on color filters for colored flatpanel displays, touch screens, electromagnetic interference,electromagnetic shielding, functional glasses (e.g., for electrochromicwindows), optoelectronic devices including EL lamps and photovoltaiccells, and the like. In addition, the transparent conductors herein canbe used in flexible devices, such as flexible displays and touchscreens. See, co-pending application Ser. No. 11/871,767.

EXAMPLES Example 1 Synthesis of Silver Nanowires

Silver nanowires were synthesized by a reduction of silver nitratedissolved in ethylene glycol in the presence of poly(vinyl pyrrolidone)(PVP). The method was described in, e.g. Y. Sun, B. Gates, B. Mayers, &Y. Xia, “Crystalline silver nanowires by soft solution processing”,Nanolett, (2002), 2(2) 165-168. Uniform silver nanowires can beselectively isolated by centrifugation or other known methods.

Alternatively, uniform silver nanowires can be synthesized directly bythe addition of a suitable ionic additive (e.g., tetrabutylammoniumchloride) to the above reaction mixture. The silver nanowires thusproduced can be used directly without a separate step of size-selection.This synthesis is described in more detail in U.S. ProvisionalApplication No. 60/815,627, in the name of Cambrios TechnologiesCorporation, the assignee of the present application, which applicationis incorporated herein in it entirety.

In the following examples, silver nanowires of 70 nm to 80 nm in widthand about 8 μm-25 μm in length were used. Typically, better opticalproperties (higher transmission and lower haze) can be achieved withhigher aspect ratio wires (i.e. longer and thinner).

Example 2 Preparation of Composite Transparent Conductors

The metal nanowires can be formulated into an ink composition prior todeposition on a substrate or a continuous conductive film such as ITOfilm, and conductive polymer films.

An ITO film can be directly sputtered on a substrate followed by thedeposition of the nanowire layer. Alternatively, a nanowire layer can befirst deposited on a substrate, followed by sputtering an ITO filmdirectly on the nanowire layer.

If the secondary conductive medium includes carbon nanotubes, the carbonnanotubes can be formulated into the same ink composition with the metalnanowires for co-deposition. Alternatively, the carbon nanotubes can beformulated into a separate ink composition for serial deposition beforeor after the deposition of the metal nanowires.

Typically, the ink composition comprises agents that facilitatedispersion of the nanostructures and/or immobilization of thenanostructures on the substrates. These agents include surfactants,viscosity modifiers, and the like. Detailed description of formulatingthe ink compositions can be found in co-pending U.S. patent applicationSer. No. 11/504,822, which is incorporated herein by reference in itsentirety.

A typical ink composition for depositing metal nanowires comprises, byweight, from 0.0025% to 0.1% surfactant (e.g., a preferred range is from0.0025% to 0.05% for Zonyl® FSO-100), from 0.02% to 4% viscositymodifier (e.g., a preferred range is 0.02% to 0.5% forhydroxypropylmethylcellulose or HPMC), from 94.5% to 99.0% solvent andfrom 0.05% to 1.4% metal nanowires. Representative examples of suitablesurfactants include Zonyl® FSN, Zonyl® FSO, Zonyl® FSH, Triton (x100,x114, x45), Dynol (604, 607), n-Dodecyl b-D-maltoside and Novek.Examples of suitable viscosity modifiers include hydroxypropyl methylcellulose (HPMC), methyl cellulose, xanthan gum, polyvinyl alcohol,carboxy methyl cellulose, hydroxy ethyl cellulose. Examples of suitablesolvents include water and isopropanol.

The ink composition can be prepared based on a desired concentration ofthe nanowires, which is an index of the loading density of the finalconductive film formed on the substrate.

The substrate can be any material onto which nanowires are deposited.The substrate can be rigid or flexible. Preferably, the substrate isalso optically clear, i.e., light transmission of the material is atleast 80% in the visible region (400 nm-700 nm).

Examples of rigid substrates include glass, polycarbonates, acrylics,and the like. In particular, specialty glass such as alkali-free glass(e.g., borosilicate), low alkali glass, and zero-expansion glass-ceramiccan be used. The specialty glass is particularly suited for thin paneldisplay systems, including Liquid Crystal Display (LCD).

Examples of flexible substrates include, but are not limited to:polyesters (e.g., polyethylene terephthalate (PET), polyesternaphthalate, and polycarbonate), polyolefins (e.g., linear, branched,and cyclic polyolefins), polyvinyls (e.g., polyvinyl chloride,polyvinylidene chloride, polyvinyl acetals, polystyrene, polyacrylates,and the like), cellulose ester bases (e.g., cellulose triacetate,cellulose acetate), polysulphones such as polyethersulphone, polyimides,silicones and other conventional polymeric films.

The ink composition can be deposited on the substrate according to, forexample, the methods described in co-pending U.S. patent applicationSer. No. 11/504,822.

As a specific example, an aqueous dispersion of silver nanowires, i.e.,an ink composition was first prepared. The silver nanowires were about35 nm to 45 nm in width and around 10 μm in length. The ink compositioncomprises, by weight, 0.2% silver nanowires, 0.4% HPMC, and 0.025%Triton x100. The ink was then spin-coated on glass at a speed of 500 rpmfor 60 s, followed by post-baking at 50° C. for 90 seconds and 180° for90 seconds. The coated film had a resistivity of about 20 ohms/sq, witha transmission of 96% (using glass as a reference) and a haze of 3.3%.

As understood by one skilled in the art, other deposition techniques canbe employed, e.g., sedimentation flow metered by a narrow channel, dieflow, flow on an incline, slit coating, gravure coating, microgravurecoating, bead coating, dip coating, slot die coating, and the like.Printing techniques can also be used to directly print an inkcomposition onto a substrate with or without a pattern. For example,inkjet, flexoprinting and screen printing can be employed.

It is further understood that the viscosity and shear behavior of thefluid as well as the interactions between the nanowires may affect thedistribution and interconnectivity of the nanowires deposited.

Example 3 Evaluation of Optical and Electrical Properties of TransparentConductors

The composite transparent conductors prepared according to the methodsdescribed herein were evaluated to establish their optical andelectrical properties.

The light transmission data were obtained according to the methodologyin ASTM D1003. Haze was measured using a BYK Gardner Haze-gard Plus. Thesurface resistivity was measured using a Fluke 175 True RMS Multimeteror contactless resistance meter, Delcom model 717B conductance monitor.A more typical device is a 4 point probe system for measuring resistance(e.g., by Keithley Instruments).

The interconnectivity of the nanowires and an areal coverage of thesubstrate can also be observed under an optical or scanning electronmicroscope.

Example 4 Evaluation of Resistivity of Composite Transparent Conductor

An ITO film was first sputtered on a glass substrate. The ITO film wasabout 250 Ω/□. A silver nanowire film was coated on the ITO film. Thesilver nanowire film was based on an ink composition that producedconductive films of about 300-500 Ω/□ in sheet resistance.

If the composite transparent conductor was treated merely as a pair ofparallel resistors, the sheet resistance would have had an expectedvalue of about 135-170 Ω/□. However, the resulting composite film showedsheet resistance in the range of 50-80 Ω/□, which was about 100% moreconductive than the expected value. Accordingly, the combined conductivemedia exhibited superior conductivity than the mere additive effects ofthe individual conductive medium.

All of the above U.S. patents, U.S. patent application publications,U.S. patent applications, foreign patents, foreign patent applicationsand non-patent publications referred to in this specification and/orlisted in the Application Data Sheet, are incorporated herein byreference, in their entirety.

From the foregoing it will be appreciated that, although specificembodiments of the invention have been described herein for purposes ofillustration, various modifications may be made without deviating fromthe spirit and scope of the invention. Accordingly, the invention is notlimited except as by the appended claims.

The invention claimed is:
 1. A layered composite transparent conductorcomprising: a first layer of a primary conductive medium comprising afirst type of electrically conductive nanostructures, wherein: aplurality of the first type of electrically conductive nanostructuresare interconnected to define a network, and the first type ofelectrically conductive nanostructures are metal nanowires or metalnanotubes; and a second layer of a secondary conductive mediumcontacting the first layer of the primary conductive medium, wherein:the secondary conductive medium comprises a second type of electricallyconductive nanostructures, the secondary conductive medium forms aconductive layer that is vertically conductive so as to equalize anelectrical potential distribution in the layered composite transparentconductor, and the second type of electrically conductive nanostructureshave a different material, dimension, geometry or structure than thefirst type of electrically conductive nanostructures.
 2. The layeredcomposite transparent conductor of claim 1, further comprising asubstrate layer, wherein: the first layer of the primary conductivemedium is positioned on top of the second layer of the secondaryconductive medium, and the second layer of the secondary conductivemedium is positioned on top of the substrate layer.
 3. The layeredcomposite transparent conductor of claim 1, further comprising asubstrate layer, wherein the first layer of the primary conductivemedium is positioned between the second layer of the secondaryconductive medium and the substrate layer.
 4. The layered compositetransparent conductor of claim 1, wherein the first layer of the primaryconductive medium and the second layer of the secondary conductivemedium are electrically coupled.
 5. The layered composite transparentconductor of claim 1, wherein the layered composite transparentconductor has a light transmission higher than 85% and a sheetresistance of less than 1000 Ω/□.
 6. The layered composite transparentconductor of claim 1, wherein the first type of electrically conductivenanostructures are silver nanowires and the second type of electricallyconductive nanostructures comprise carbon nanotubes.
 7. A devicecomprising a layered composite transparent conductor comprising: a firstlayer of a primary conductive medium comprising a first type ofelectrically conductive nanostructures, wherein: a plurality of thefirst type of electrically conductive nanostructures are interconnectedto define a network, and the first type of electrically conductivenanostructures are metal nanowires or metal nanotubes; and a secondlayer of a secondary conductive medium contacting the first layer of theprimary conductive medium, wherein: the secondary conductive mediumcomprises a second type of electrically conductive nanostructuredifferent than the first type of electrically conductive nanostructure,and the secondary conductive medium forms a conductive layer that isvertically conductive so as to equalize an electrical potentialdistribution in the layered composite transparent conductor.
 8. Thedevice of claim 7, wherein the first type of electrically conductivenanostructures are silver nanowires.
 9. The device of claim 7, whereinthe layered composite transparent conductor forms a first transparentelectrode.
 10. The device of claim 9, further comprising a secondcomposite transparent conductor forming a second transparent electrodeopposite to the first transparent electrode.
 11. The device of claim 7,wherein the layered composite transparent conductor has a lighttransmission higher than 85% and a sheet resistance of less than 1000Ω/□.
 12. The device of claim 7, wherein the device is a flat paneldisplay, a touch screen, an electromagnetic shielding device, anelectromagnetic interference device, an electroluminescent device or aphotovoltaic cell.
 13. The layered composite transparent conductor ofclaim 1, wherein the second type of electrically conductivenanostructures comprise a plurality of carbon nanotubes.
 14. The layeredcomposite transparent conductor of claim 13, further comprising a thirdtype of electrically conductive nanoparticles, wherein the third type ofelectrically conductive nanoparticles are metal nanoparticles, carbonblack nanoparticles, graphene sheets, or a combination thereof.
 15. Thelayered composite transparent conductor of claim 1, wherein theplurality of the first type of electrically conductive nanostructuresare above an electrical percolation threshold.
 16. The layered compositetransparent conductor of claim 1, wherein the second type ofelectrically conductive nanostructures are nanoparticles.
 17. Thelayered composite transparent conductor of claim 1, wherein the firsttype of electrically conductive nanostructures have a first diameter andthe second type of electrically conductive nanostructures have a seconddiameter different than the first diameter.
 18. The layered compositetransparent conductor of claim 17, wherein the second diameter is lessthan the first diameter.
 19. The layered composite transparent conductorof claim 1, wherein the first type of electrically conductivenanostructures have a first aspect ratio and the second type ofelectrically conductive nanostructures have a second aspect ratiodifferent than the first aspect ratio.